Photographic light-sensitive materials are generally composed of light-sensitive photographic emulsion layers and light insensitive layers such as an interlayer, an emulsion protective layer, a filter layer, or an antihalation layer applied, directly or indirectly through a subbing layer, to one side or both sides of the support consisting of, for example, an .alpha.-olefin such as polyethylene, a cellulose ester such as cellulose acetate or nitrocellulose, a polyester such as polyethylene terephthalate or polyethylene napththalate, polystyrene, paper, or a synthetic paper. In light-sensitive materials such as color photographic elements, auxiliary layers such as an antistatic layer, a curl preventing layer, a magnetic recording layer, a barrier layer, a scratch resistant overcoat layer, or a surface lubricant layer, are provided on the backside of the support in order to enhance the photographic or physical quality of the photographic light-sensitive materials.
It is always desirable to have a backside protective overcoat that serves as many functions as possible in order to reduce manufacturing complexity and cost. It is also desirable to have such a layer formed by coating and drying from coating compositions based on solvents that are less hazardous to the environment.
Prior art has disclosed the use of a protective overcoat or a "barrier" layer to maintain post-process conductivity of an antistatic layer. Typically such protective overcoats consist of hydrophobic materials such as cellulose acetates, cellulose acetate butyrates, cellulose acetate propionates, cellulose nitrates, polyacrylates, polymethacrylates, polystyrene, and poly(vinyl acetal).
When such hydrophobic barrier layers are used as an outermost surface layer, deposition of material or "scum" formation on the outermost surface following photographic processing is commonly seen. For example, U.S. Pat. No. 4,735,976, incorporated by reference herein, discusses how surfactant from the final photographic processing solution, known as the stabilizer solution, can form a deposit on the outermost surface layer and thereby lead to an objectionable surface haze or scum. Similarly, U.S. Pat. No. 4,582,784, incorporated by reference herein, discusses the occurrence of spotted drying unevenness on the outermost surface. Another type of processing scum that is particularly troublesome is hard-water scum. Processing laboratories that are located in hard-water areas are particularly susceptible to this problem. After processing in solutions prepared using hard-water, a white hazy surface scum, sometimes uniform and sometimes more liney and streaky, can be seen on the film. Chemical analysis of the hard-water scum typically reveals hard-water salts of calcium, magnesium, and sodium.
Such surface deposits can impact the physical performance of the element in a variety of ways. For example, large deposits of material on a photographic film lead to readily visible defects on photographic prints or are visible upon display of motion picture film. Alternatively, post-processing debris can influence the ability of a processed film to be overcoated with an ultraviolet curable abrasion resistant layer, as is done in professional photographic processing laboratories employing materials such as PhotoGard, 3M. Finally, processing residue on photographic elements can impact the ability to read or write magnetically recorded information on a processed film, such as the new Advanced Photographic System film.
U.S. Pat. Nos. 4,612,279 and 4,735,976, incorporated by reference herein, describe a protective overcoat comprising a blend of cellulose nitrate and a copolymer containing acrylic or methacrylic acid for eliminating objectionable surface haze or scum formed during photographic processing. U.S. Pat. No. 4,582,784 describes an uppermost surface layer composed of a hydrophobic cellulose ester polymer and a hydrophilic vinyl polymer for reducing spotted drying unevenness. However, layer compositions disclosed in the above art do not provide adequate barrier properties and adequate resistance to mechanical scratch and high humidity ferrotyping. High humidity ferrotyping becomes a problem especially for photographic systems such as the so-called Advanced Photographic System where the processed element may be re-introduced into a cassette. Such a system allows for compact and clean storage of the processed element until such time when it may be removed for additional prints or to interface with display equipment. Storage in the roll is preferred to facilitate location of the desired exposed frame and to minimize contact with the negative. U.S. Pat. No. 5,173,739, incorporated by reference herein, discloses a cassette designed to thrust the photographic element from the cassette, eliminating the need to contact the film by mechanical or manual means. Published European Patent Application 0 476 535 A1 describes how the developed film may be stored in such a cassette. The dimensions of such a so-called thrust cassette require that the processed photographic element be wound tightly and under pressure, causing direct close contact between the front and back sides which results in ferrotyping, especially at high temperature and high relative humidity.
U.S. Pat. Nos. 5,597,680, 5,597,681, and 5,695,919, incorporated by reference herein, describe coating layer compositions comprising a dispersion of core/shell polymer particles, wherein the core portion may be impregnated with a lubricant, in a liquid organic medium. Furthermore, such coating compositions may comprise a solution polymer soluble in the desired solvent medium. Examples of solution polymers utilized in U.S. Pat. Nos. 5,597,680, 5,597,681, and 5,695,919 include polymethylmethacrylate (Elvacite 2041) and nitrocellulose. Solution polymers comprising acid functional groups are not discussed. Coating layers based on U.S. Pat. Nos. 5,597,680, 5,597,681, and 5,695,919 provide photographic elements with excellent barrier properties and excellent resistance to mechanical scratch and ferrotyping at high temperature and in moist environments. However, such coating compositions do not provide the photographic elements with adequate protection against surface haze or scum formation under more severe processing conditions.
U.S. Pat. No. 5,466,536 describes coating layers comprising copolymers of styrene, alkylstyrene, (alkyl)acrylic acid, and (alkyl)acrylic acid alkylester. Examples show aqueous coating compositions comprising aqueous dispersions of such copolymers. Typically two aqueous copolymer dispersions are used where each copolymer has a different level of acid containing monomers. Blends of higher and lower level acid containing copolymers are discussed. However, extension to an organic solvent medium and to one which contains a mixture of solvent dispersible polymer particles and a solvent soluble polymer is not anticipated based on this patent, nor the other prior art patents.
Blisters are characteristic defects that are often observed on the backside of a developed film. When viewed by reflected light, the defect appears as a circular-shaped topographic feature with a wrinkled surface texture. The diameter of the blister can be as large as several millimeters. Blisters not only affect the photographic image quality, but also the read and write ability of a magnetic layer and the image digitization, for example, by a scanner.
The objective of the present invention is to provide a photographic element with a backside protective overcoat composition that meets all of the physical and mechanical requirements as described above while avoiding the problems and limitations of the prior art.